Light-emitting diodes (LEDs), particularly white LEDs, have increased in size in order to provide the total light output needed for general illumination. As LED technology has advanced, the efficacy (measured in lumens/Watt) has gradually increased, such that smaller dies now produce as much light as was previously created by emission from far larger die areas. Nonetheless, the trend favoring higher light outputs has led to larger semiconductor LED die sizes, or, for convenience, arrays of smaller dies in series or series-parallel arrangements. Series arrangements are generally favored because the forward voltage of LEDs varies slightly, resulting, for parallel arrangements, in an uneven distribution of forward currents and, consequently, uneven light output.
For many applications, it is desirable to have a light source that produces a light beam whose angular distribution can be varied. Variability is needed, for example, to create a wide-angle light beam for illuminating an array of objects, or a narrow-angle beam for illuminating a single, small object. Conventionally, the angular distribution is varied by moving the light source(s) (e.g., the LED arrangement) toward or away from the focal point of a lens or parabolic mirror. As the light source is moved away from the focal point, its image is blurred, forming a wider beam. Unfortunately, in doing so, the image is degraded, becoming non-uniform; in the case of the familiar parabolic reflector used in flashlights, a dark “donut hole” is formed, which is visually undesirable and sacrifices full illumination of the scene. Furthermore, moving the lens often reduces the collection efficiency of the lens, as light that is not refracted by a lens or reflected by a reflector surface is lost.
Because of these optical artifacts and efficiency losses, most light sources use a single, fixed lens. For light bulbs such as, e.g., MR-16 halogen bulbs, several different types of optics are manufactured to create beams of various beam divergences, ranging from narrow beam angles (“spot lights”) to wide angles (“flood lights”), with various degrees in between. Unless the user maintains different light bulbs on hand to accommodate all potentially desired beam divergences, however, he will generally be limited to one or a small number of alternatives. Traveling with an assortment of bulbs for portable light sources is even less realistic. As a result, users often tolerate either a source ill-suited to changing or unexpected conditions, or the poor optical quality of light sources with variable beam optics. A need, therefore, exists for light sources that produce variable beam angles without sacrificing beam quality.
When LEDs are utilized as a light source, they are generally arranged in clusters or arrays to coincide with particular design specifications to provide a desired light output. The LED arrays combine the higher efficiency, increased lifetime, and reliability benefits of LEDs with the light output levels of many conventional light sources to serve the general lighting market, including street lights, commercial lighting and consumer applications. For example, with a proper fixture design, LED arrays can deliver a few times the energy efficiency of traditional lighting with a 30% reduction in cost. However, a single LED die in the LED array may fail during manufacture or operation due to the failure of, e.g., the LED die itself, a conductive path, a substrate or an electrical or mechanical connection of the LED to the conductive path; such LED failure can cause highly visible defects in the pattern of the luminous intensity and/or light uniformity, and in some cases can lead to failure of the entire LED array. It is therefore desirable to have individually addressable LEDs for mitigating the effects of a single LED failure. For example, upon detecting an LED failure, the LED driver circuitry may deactivate only the failed LED; the light output reduction resulting from the single LED deactivation may be on the order of 1%, which is generally unnoticeable to the observer. Additionally, the individually addressable LEDs may be selectively activated and deactivated, thereby providing flexible control over the overall illumination brightness and/or color. Conventional individually addressable LEDs, however, require electronic circuitry that typically occupies a significant surface area on the substrate upon which the LEDs are located; as a result, the LED packing efficiency is reduced.
Further, conventional LED arrays may suffer from a significant drop in lifetime due to inefficient dissipation of heat generated from the LEDs and/or electronic components associated therewith. This is because conventional LED arrays are preferably close-packed, i.e., have minimal spacing between the individual LED dies, on a printed circuit board (PCB) for reducing optical artifacts and providing beam uniformity. The PCB, however, has low thermal conductivity, and therefore, cannot quickly dissipate the waste heat generated during operation of the LED. Further, despite the desirability of LED close-packing, the spacing between conventional LED dies on the PCB can only be minimized to a certain degree because of minimum distance requirements between components on a PCB. For example, close-packed LED dies on a PCB typically have a distance of roughly 100 μm therebetween for wiring conducting line traces on the PCB.
Although it is feasible to include multiple conducting levels in a single PCB, thereby embedding the conducting traces (e.g., copper traces) below the LEDs without using “real estate” on the PCB surface alongside of the LED, the copper vias that extend through the PCB to contact the LEDs take up valuable area on the PCB surface; this, again, creates a spacing limitation between closely-packed LEDs. Such spatial resolution limitations, unfortunately, cannot easily be overcome by improving the LED package technology because it is a fundamental limitation of the predominant patterning approach used in currently available PCB fabrication technology.
Consequently, there is a need for an approach that can provide individually addressable LEDs without requiring valuable area on the substrate surface for the LED driver/support circuitry. Ideally, such an approach would provide effective close-packing of the LEDs while efficiently dissipating heat generated during LED operation.